3. 3
Objectives
Approximate a definite integral using the Trapezoidal
Rule.
Approximate a definite integral using Simpson’s Rule.
Analyze the approximate errors in the Trapezoidal Rule
and Simpson’s Rule.
5. 5
One way to approximate a definite integral is to use
n trapezoids, as shown in Figure 4.42.
In the development of this method,
assume that f is continuous and
positive on the interval [a, b].
So, the definite integral
represents the area of the region
bounded by the graph of f and the
x-axis, from x = a to x = b.
Figure 4.42
The Trapezoidal Rule
6. 6
First, partition the interval [a, b] into n subintervals, each of
width Δx = (b – a)/n, such that
Then form a trapezoid for each
subinterval (see Figure 4.43).
Figure 4.43
The Trapezoidal Rule
7. 7
The Trapezoidal Rule
The area of the ith trapezoid is
This implies that the sum of the areas of the n trapezoids
8. 8
The Trapezoidal Rule
Letting you can take the limits as to
obtain
The result is summarized in the following theorem.
10. 10
Example 1 – Approximation with the Trapezoidal Rule
Use the Trapezoidal Rule to approximate
Compare the results for n = 4 and n = 8, as shown in
Figure 4.44.
Figure 4.44
11. 11
Example 1 – Solution
When n = 4, Δx = π/4, and you obtain
12. 12
Example 1 – Solution
When and you obtain
cont’d
14. 14
Simpson’s Rule
One way to view the trapezoidal approximation of a definite
integral is to say that on each subinterval you approximate f
by a first-degree polynomial.
In Simpson’s Rule, named after the English mathematician
Thomas Simpson (1710–1761), you take this procedure
one step further and approximate f by second-degree
polynomials.
Before presenting Simpson’s Rule, we list a theorem for
evaluating integrals of polynomials of degree 2 (or less).
16. 16
Simpson’s Rule
To develop Simpson’s Rule for approximating a definite
integral, you again partition the interval [a, b] into
n subintervals, each of width Δx = (b – a)/n.
This time, however, n required to be even, and the
subintervals are grouped in pairs such that
On each (double) subinterval [xi – 2, xi ], you can
approximate f by a polynomial p of degree less than or
equal to 2.
17. 17
Simpson’s Rule
For example, on the subinterval [x0, x2], choose the
polynomial of least degree passing through the points
(x0, y0), (x1, y1), and (x2, y2), as shown in Figure 4.45.
Figure 4.45
18. 18
Simpson’s Rule
Now, using p as an approximation of f on this subinterval,
you have, by Theorem 4.18,
Repeating this procedure on the entire interval [a, b]
produces the following theorem.
23. 23
Error Analysis
When you use an approximation technique, it is important
to know how accurate you can expect the approximation to
be.
The following theorem gives the formulas for estimating the
errors involved in the use of Simpson’s Rule and the
Trapezoidal Rule.
In general, when using an approximation, you can think of
the error E as the difference between and the
approximation.
25. 25
Example 3 – The Approximate Error in the Trapezoidal Rule
Determine a value of n such that the Trapezoidal Rule will
approximate the value of with an error that is
less than or equal to 0.01.
Solution:
Begin by letting and finding the second
derivative of f.
The maximum value of |f''(x)| on the interval [0, 1] is
|f''(0)| = 1.
26. cont’d
26
Example 3 – Solution
So, by Theorem 4.20, you can write
To obtain an error E that is less than 0.01, you must choose
n such that 1/(12n2) ≤ 1/100.
27. 27
So, you can choose n = 3 (because n must be greater than
or equal to 2.89) and apply the Trapezoidal Rule, as shown
in Figure 4.46, to obtain
Figure 4.46
Example 3 – Solution
cont’d
28. 28
Example 3 – Solution
So, by adding and subtracting the error from this estimate,
you know that
cont’d
